The sub-base layer is the main load-bearing layer of a pavement. It is designed to evenly spread the load of the paving, and any traffic thereon, to the sub-grade below. A well-constructed sub-base will prevent settlement and channelization - the phenomenon common on cheap installations of block paving, where two 'ruts' develop in the paving. These 'ruts' are caused when a car travels over the same line of paving to the garage, every morning and every night. The sub-base is intended to prevent channelization and settlement.
All too often, cut-price residential block-paving installations omit the sub-base as a major cost-saver. Householders are unlikely to be aware of the need for such a layer, and, if out all day when the work is being done, may never notice its absence. A common tale I hear is....."Such-and-such-a body said we didn't need any stone, because the dirt under the old flagstones will be firm enough!" – We usually get this tale once the paving has sunk, and the original contractor is long gone.
When discussing the construction of driveways with clients, we liken the sub-base to a carpet underlay...you can spend a fortune on the finest carpet, but if you don't use an underlay, it's never going to look or feel right. If a contractor tells you that a sub-base isn't required beneath your new drive, then ask how the sub-grade (soil beneath drive) is going to spread the load of the paving and what guarantee is offered with regard to settlement.
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